Mode Control and Dynamic Population Gratings in Quantum-Dot Lasers

Abstract

Single-mode operation is essential for integrated semiconductor lasers, yet most solutions rely on regrowth, etched gratings, or other complex fabrication steps that limit scalability. We show that quantum-dot (QD) lasers can achieve stable single-mode lasing through a simple cavity design using dynamic population gratings (DPGs). Owing to the low lateral carrier diffusion of QDs, a strong standing-wave-induced carrier grating forms in a reverse-biased saturable absorber and provides self-aligned, mode-selective feedback not attainable in quantum-well devices. A single-ring laser achieves 46 dB side-mode suppression ratio (SMSR), while a dual-ring Vernier laser delivers ($>$ 46 nm) tuning range and up to 52.6 dB SMSR, with continuous-wave operation up to $80\,^{\circ}\mathrm{C}$. The laser remains single-mode under $-10.6$ dB external optical feedback and supports isolator-free data transmission at 32 Gbps. These results establish DPG-enabled QD lasers as a simple and scalable route to tunable, feedback-resilient on-chip light sources for communication, sensing, and reconfigurable photonic systems.

Figures (4)

• Figure 1: Device concept, structure, and principle of operation. (a) Schematic of the dual-ring laser with spatially separated gain section (gray) and reverse-biased SA section (red). The SA introduces nonlinear absorption for mode suppression (I), the detuned rings provide Vernier filtering for coarse selection (II), and the standing-wave–induced DPG supplies reflective feedback that reinforces the dominant mode (III). SA, saturable absorber; ISO, isolation; SOA, semiconductor optical amplifier; DPG, dynamic population grating. (b) Illustration of the DPG formation by counter-propagating waves in the SA regions, where the SA enhances the standing-wave induced carrier modulation. (c) Lateral carrier diffusion and (d) Calculated carrier-grating profiles in QD and QW active media. (e) Brief fabrication process flow of QD laser.
• Figure 2: Device performance of the dual-ring QD laser with DPG. (a) Optical micrograph of the fabricated tunable laser array, consisting of nine devices on a 5 mm × 1.05 mm bar. Inset: SEM image of a single dual-ring laser. (b) LIV curves measured at various chip temperatures, confirming continuous-wave lasing up to $80\,^{\circ}\mathrm{C}$. (c) Representative single-mode spectrum with SMSR exceeding 52.6 dB. (d) Measured tuning characteristics, demonstrating a wavelength tuning range of 4̃6 nm. (e) Extracted SMSR and peak power as a function of lasing wavelength.
• Figure 3: Feedback dynamics and isolator-free transmission of the QD laser. (a) Experimental setup of feedback dynamics and isolator-free transmission link. (b) Spectrogram of the QD and QW lasers under increasing external optical feedback, showing robust single-mode operation of the QD laser up to $-10.6$ dB feedback. (c) RF spectra of the QD and QW lasers under the same conditions, confirming the absence of coherence collapse of the QD laser. (d) Isolator-free emission spectra of the QD tunable laser at five representative wavelengths. (e) Eye diagrams recorded at 32 Gbps for five representative wavelengths of the QD laser without an isolator, showing clear and open eyes with error-free operation.
• Figure 4: Single-ring and dual-ring lasers with HWC and SA-assisted DPG operation. (a) Voltage-controlled spectral evolution of the single-ring laser at 40 mA, showing the emergence of a stable single-mode window confirmed by the extracted SMSR and peak-power curves. (b) Voltage-dependent lasing spectra of the dual-ring Vernier laser with the currents of the two rings and the SOA are fixed at $22~\mathrm{mA}$, $26~\mathrm{mA}$, and $60~\mathrm{mA}$, respectively.